This invention is in the field of optoelectronic devices, and specifically relates to the coupling of driving signals to semiconductor devices for modulating optical signals.
Semiconductor modulators for optical signals are extensively used in various applications, particularly in the field of telecommunications. One type of semiconductor modulator is an electroabsorption modulator (EAM). In order to operate an EAM, a variable voltage (RF signal) is provided across the terminals of the EAM. The voltage dependency of the absorption of the EAM at a selected operating wavelength results in a modulated optical signal. In practice it is often desirable for a DC bias voltage to be applied to the EAM as well. Adjustment of the DC bias voltage may allow tuning of the EAM performance and/or tuning of the operating wavelength of the EAM.
FIG. 1A illustrates a prior art EAM circuit employing DC coupled drive electronics represented by an RF source. Drive electronics 100 are represented as voltage source 102 coupled across a resistor 104. Drive electronics 100 are coupled to hybrid integrated circuit (HIC) assembly, or packaging, 106. HIC 106 is shown as a transmission line 108 and an inductor 110 to represent the connection between the transmission line 108 and EAM 112. EAM 112 is represented as resistor 114 and diode 116 in series, with a voltage controlled current source 118 representing the photocurrent, and pad capacitance 120 in parallel. Termination 122 includes a resistor 126, with an inductor 124 representing the connection to the EAM 112. The EAM has an n-type semiconductor side and a p-type semiconductor side. The n-side of EAM 112 is connected to a source of reference potential 136 (e.g. ground), while the p-side is coupled to the drive electronics through HIC 106. Any DC offset voltage provided to EAM 112 must be supplied by drive electronics 100. The need to provide the DC offset voltage may strain the drive electronics and lead to early component failure.
An alternative prior art circuit is shown with reference to FIG. 1B. In this circuit the n-side of the EAM is also connected to ground 136. This circuit employs a bias tee circuit 128 to connect drive electronics 100 to EAM 112. A DC bias to EAM 112 may be provided through bias tee circuit 128 by DC voltage supply 134. The use of bias tee circuit 128 in the circuit of FIG. 1B permits the DC offset voltage to be set with precision, and, compared to the prior art circuit shown in FIG. 1A, has less load on drive electronics 100. However, a suitable bias tee for high speed applications is relatively large, generally much larger than the EAM package itself, and adds significantly to the cost of a package incorporating drive electronics, the EAM, and other related electronics.
One embodiment of the present invention is drive circuitry to provide a DC bias voltage and a high frequency modulation current to an electroabsorption modulator (EAM), which includes a first semiconductor type contact and an second semiconductor type contact. The drive circuitry includes a high frequency modulation current source, a coupling capacitor, and a first DC lead. The first modulation lead of the high frequency modulation current source is electrically coupled to the first semiconductor type contact of the EAM and the second modulation lead of the high frequency modulation current source is electrically coupled to an AC ground. The coupling capacitor includes a EAM-side capacitor electrode which is electrically coupled to the second semiconductor type contact of the EAM, a non-EAM-side capacitor electrode which is electrically coupled to the AC ground, and a dielectric layer which is disposed between the EAM-side capacitor electrode and the non-EAM-side capacitor electrode. The first DC lead is electrically coupled to the EAM-side capacitor electrode and configured to be coupled to a first DC potential.
Another embodiment of the present invention is a monolithic EAM and coupling capacitor. The monolithic EAM and coupling capacitor include a substrate with a top surface. A non-EAM-side capacitor electrode is coupled to the top surface of the substrate, a capacitor dielectric layer is coupled to the non-EAM-side capacitor electrode and an EAM-side capacitor electrode is coupled to the capacitor dielectric layer to form the coupling capacitor. An EAM base layer is formed of a first type semiconductor material. This EAM base layer is electrically coupled to the EAM-side capacitor electrode. An EAM waveguide, which includes an electroabsorption portion, is formed on the EAM base layer. An EAM second type semiconductor layer is formed on the EAM waveguide and an EAM electrode is electrically coupled to the EAM second type semiconductor layer.
A further embodiment of the present invention is an alternative monolithic EAM and coupling capacitor. The alternative monolithic EAM and coupling capacitor includes a substrate formed of a first type semiconductor material with a top surface and a bottom surface. An EAM-side capacitor electrode is coupled to the bottom surface of the substrate, a capacitor dielectric layer is coupled to the EAM-side capacitor electrode and a non-EAM-side capacitor electrode is coupled to the capacitor dielectric layer to form the coupling capacitor. An EAM waveguide, which includes an electroabsorption portion, is formed on the top surface of the substrate. An EAM second type semiconductor layer is formed on the EAM waveguide and an EAM electrode is electrically coupled to the EAM second type semiconductor layer.
Yet another embodiment of the present invention is a method of manufacturing a monolithic EAM and coupling capacitor. A substrate formed of a first type semiconductor material with a top surface and a bottom surface is provided. An EAM waveguide layer, which includes an electroabsorption portion, is formed on the top surface of the substrate. An EAM second type semiconductor layer in formed on the EAM waveguide. The EAM second type semiconductor layer and the EAM waveguide layer are etched to form an EAM second type semiconductor region and an EAM waveguide. An EAM electrode is formed on the EAM second type semiconductor region. An EAM-side capacitor electrode is formed on the substrate. A capacitor dielectric layer, which is electrically coupled to the EAM-side capacitor electrode, is formed and a non-EAM-side capacitor electrode is formed on the capacitor dielectric layer.
A still further embodiment of the present invention is an additional monolithic EAM and coupling capacitor. The additional monolithic EAM and coupling capacitor includes a substrate, including a first type semiconductor material portion having a top surface. An EAM electrode is electrically coupled to the first type semiconductor material portion of the substrate. An EAM waveguide is formed on the top surface of the first type semiconductor material portion of the substrate and includes an electroabsorption portion. An EAM second type semiconductor layer is formed on the EAM waveguide. An EAM-side capacitor electrode is electrically coupled to the EAM second type semiconductor layer, a capacitor dielectric layer is formed on the EAM-side capacitor electrode, and a non-EAM-side capacitor electrode formed on the capacitor dielectric layer.
An additional embodiment of the present invention is an alternative method of manufacturing a monolithic EAM and coupling capacitor. A substrate including a first type semiconductor material portion having a top surface is provided. An EAM waveguide layer, which includes an electroabsorption portion, is formed on the top surface of the first type semiconductor material portion of the substrate. An EAM second type semiconductor layer is formed on the EAM waveguide layer. The EAM second type semiconductor layer and the EAM waveguide layer are etched to form an EAM second type semiconductor region and an EAM waveguide. An EAM electrode is formed on the first type semiconductor material portion of the substrate. An EAM-side capacitor electrode is formed on the EAM second type semiconductor region, a capacitor dielectric layer is formed on the EAM-side capacitor electrode, and a non-EAM-side capacitor electrode is formed on the capacitor dielectric layer.
Yet a further embodiment of the present invention is a method of manufacturing a monolithic co-sided EAM and coupling capacitor. A non-conducting substrate with a top surface is provided. A co-sided EAM is formed on the top surface of the non-conducting substrate. Formation of the co-sided EAM includes the steps of: forming an EAM first type base layer with a top surface on the top surface of the non-conducting substrate; forming an EAM waveguide layer on the EAM first type base layer, the EAM waveguide layer including an electroabsorption portion; forming an EAM second type semiconductor layer on the EAM waveguide; etching the EAM second type semiconductor layer and the EAM waveguide layer to form an EAM second type semiconductor region and an EAM waveguide and exposing at least one side portion of the top surface of the EAM first type base layer; forming an EAM insulating layer on the at least one side portion of the top surface of the EAM first type base layer; etching the EAM insulating layer to expose at least one contact region of the at least one side portion of the top surface of the EAM first type base layer. At least one capacitor is also formed on the top surface of the non-conducting substrate. Formation of each capacitor includes the steps of: forming a non-EAM-side capacitor electrode on the top surface of the non-conducting substrate; forming a capacitor dielectric layer on the non-EAM-side capacitor electrode; and forming an EAM-side capacitor electrode on the capacitor dielectric layer.